GaN Decomposition Research Articles

Properties Investigation and Damage Analysis of GaN Photoconductive Semiconductor Switch Based on SiC Substrate.

The GaN photoconductive semiconductor switches (PCSSs) with low leakage current and large on-state current are suitable for several applications, including fast switching and high-power electromagnetic pulse equipment. This paper demonstrates a high-power GaN lateral PCSS device. An output peak current of 142.2 A is reached with an input voltage of 10.28 kV when the GaN lateral PCSS is intrinsically triggered. In addition, the method of retaining the AlGaN/GaN heterostructure between electrodes on PCSSs is proposed, which results in increasing the output peak current of the PCSS. The damage mechanism of the PCSS caused by a high electric field and high excitation laser energy is analyzed. The obtained results show that the high heat generated by the large current leads to the decomposition of GaN, and thus, the Ga forms a metal conductive path, resulting in the failure of the device.

Pros and Cons of (NH4)2S Solution Treatment of p-GaN/Metallization Interface: Perspectives for Laser Diode.

The impact of wet treatment using an (NH4)2S-alcohol solution on the interface state of the p-GaN/Ni/Au/Pt contact system and laser diode processing was investigated. Sulfur wet cleaning resulted in reduced surface roughness and contact resistivity. The lowest specific contact resistance (ρc < 1 × 10-4 Ω·cm2) was achieved with samples treated with an (NH4)2S-isopropanol solution, whereas the highest resistivity (ρc = 3.3 × 10-4 Ω·cm2) and surface roughness (Ra = 16 nm) were observed in samples prepared by standard methods. Annealing the contact system in an N2 + O2 + H2O atmosphere caused degradation through species inter-diffusion and metal-metal solid solution formation, irrespective of the preparation method. Standard prepared substrates developed a thin GaN-Au intermediate layer at the interface after heat treatment. Enhanced adhesion and the absence of GaN decomposition were observed in samples additionally cleaned with the (NH4)2S-solvent solution. Complete oxidation of nickel to NiO was observed in samples that underwent additional sulfur solution treatment. The intensity of metal species mixing and nickel oxidation was influenced by the metal diffusion rate and was affected by the initial state of the GaN substrate obtained through different wet treatment methods.

Decomposition behavior of GaN under solid and ambient N2 pressure

We investigated the decomposition behavior of GaN crystals under high N2 and solid pressures using a belt-type high-pressure apparatus. The decomposition curve of GaN under solid pressure showed a considerably moderate increase compared to that under a N2 pressure. Such difference was attributed to the N2 dissolution in the Ga melt, where a high N2 pressure creates a supersaturated GaN state, which effectively prevent its decomposition. Conversely, under a solid pressure, the increase in the GaN decomposition temperature followed a common change in enthalpy, which is consistent with its typical thermodynamic behavior. The decomposition behavior of GaN under a solid pressure demonstrated its successful high-temperature annealing using a belt-type high-pressure apparatus. Therefore, this study provides valuable insights into the thermodynamic stability of GaN under different pressures, which is critical for optimizing high-performance GaN-based devices.

Controlled nano-roughening of the GaN surface by post-growth thermal annealing

GaN-based semiconductor structures have been widely used in photonic, electronic, and optoelectronic devices. However, due to the lack of lattice-matched substrates for GaN, several schemes related to material growth and device structure, including surface roughening, have been proposed to improve the efficiency of GaN-based devices. In our work, we developed an experimental approach to achieving a controllable roughening of the GaN epilayer surface by thermal post-annealing. The degree of surface roughness, i.e., nanometric modification, is controlled by the number of annealing cycles. The samples were annealed at 1200 °C under N2 ambient. Under this condition, the first stage of GaN decomposition occurs. This annealing process led to the formation of Ga-rich GaN nanoparticles at the film surface with no significant change in the dislocation density. Furthermore, this treatment greatly decreases the compressive stress degree, resulting in a considerable improvement in the optical properties of the GaN samples.

2D-Material-Assisted GaN Growth on GaN Template by MOCVD and Its Exfoliation Strategy.

The production of freestanding membranes using two-dimensional (2D) materials often involves techniques such as van der Waals (vdW) epitaxy, quasi-vdW epitaxy, and remote epitaxy. However, a challenge arises when attempting to manufacture freestanding GaN by using these 2D-material-assisted growth techniques. The issue lies in securing stability, as high-temperature growth conditions under metal-organic chemical vapor deposition (MOCVD) can cause damage to the 2D materials due to GaN decomposition of the substrate. Even when GaN is successfully grown using this method, damage to the 2D material leads to direct bonding with the substrate, making the exfoliation of the grown GaN nearly impossible. This study introduces an approach for GaN growth and exfoliation on 2D material/GaN templates. First, graphene and hexagonal boron nitride (h-BN) were transferred onto the GaN template, creating stable conditions under high temperatures and various gases in MOCVD. GaN was grown in a two-step process at 750 and 900 °C, ensuring exfoliation in cases where the 2D materials remained intact. Essentially, while it is challenging to grow GaN on 2D material/GaN using only MOCVD, this study demonstrates that with effective protection of the 2D material, the grown GaN can endure high temperatures and still be exfoliated. Furthermore, these results support that vdW epitaxy and remote epitaxy principle are not only possible with specific equipment but also applicable generally.

Palladium-Based Contacts on p-GaN and Their Application in Laser Diodes.

In this paper, we investigate the effect of Pd thickness and heat treatment on Pd/Ni/Au/p-GaN metal contacts. The as-deposited samples exhibit a smooth morphology and non-linear I-V characteristics. Heat treatment in a N2 atmosphere leads to degradation of the contact microstructure, resulting in diffusion of Ga, void formation on the interface and mixing of metals. Annealing in a mixture of N2 and O2 improves adhesion and reduces contact resistance. However, this process also induces GaN decomposition and species mixing. The mixing of metal-Ga and metal-metal remains unaffected by the method of thermal treatment but depends on gas composition for thin Pd contacts. To achieve low-resistance contacts (≈1 × 10-4 Ω cm2), we found that increasing the Pd thickness and using N2 + O2 as the annealing environment are effective measures. Nevertheless, the degradation effect of the annealed contact microstructure in the form of the void generation becomes evident as the thickness of Pd increases. Laser diodes (LDs) with optimized palladium-based contacts operate at a voltage of 4.1 V and a current density of 3.3 kA/cm².

Reducing GaN crystal dislocations through lateral growth on uneven seed crystal surfaces using the Na-flux method

In recent years, we have achieved low threading dislocation density in GaN wafers by using the Na-flux multi-point seed technique. However, the resulting wafers exhibit regions of high dislocation density, exceeding 105 cm−2 at the coalescence boundary where pyramidal crystals merge. In this study, we discovered that annealing seed crystals at 900 °C generated an uneven surface with approximately 50 μm of GaN decomposition, and growing GaN on the thus-generated uneven surfaces induced lateral growth composed of facets. We then investigated the effect of the uneven surface on dislocation reduction and found that the average threading dislocation density of the grown crystal was reduced from cm−2 in the seed crystal to cm−2. We confirmed that the reduction in threading dislocation density was due to the termination of dislocations by means of inclusions, and to mergers or annihilation as they encountered one another during facet growth.

A novel multi-gradient evolutionary deep learning approach for few-shot wind power prediction using time-series GAN

Due to the lack of historical data, accurate prediction is a great challenge for newly constructed wind farms (NWFs). How to guarantee satisfactory prediction accuracy with limited data has become a bottleneck issue in wind power prediction (WPP). To solve the few-shot learning problem in NWFs, a novel multi-gradient evolutionary deep learning neural network (EATDLNN) prediction model is proposed, which incorporates the time-series GAN (TimeGAN) and multivariate variational mode decomposition (MVMD) method. TimeGAN adds an auto-encoder network and a supervised loss function to GAN, which enables it to capture the dynamic time series correlation characteristics of wind data and generate high-quality samples. MVMD is then employed to simultaneously decompose nonstationary sequences. In the EATDLNN prediction model, an evolution-based multi-gradient training approach is proposed to train the DLNN by integrating three gradient descent methods (i.e., Adam, MBGD-Momentum, RMSprop). Compared with the traditional single-gradient training approach, the multi-gradient training approach provides different gradient descent paths and the gradient can adaptively choose to update towards the path with the smallest error based on the evolutionary framework, which can help the model jump out of the local optimum and enhance the generalization ability. Massive experiments prove the excellent performance of the proposed method, especially in one-step prediction, the root mean square errors reduce by 79.40%∼91.05% compared with other state-of-the-art methods.

Avoiding Gallium Pollution in Close‐Coupled Showerhead Reactors, Alternative Process Routes

In contrast to the previous work which solved the problem of gallium pollution using hardware modifications, herein, the changes are examined to process conditions to reduce gallium pollution in InAlN layer. Using a model of GaN decomposition followed by gallium desorption and diffusion to the showerhead, different process conditions are used to limit either the desorption or the diffusion. Reducing the GaN growth temperature gives some improvement by reducing desorption, but affects channel mobility, likely due to increased carbon incorporation. Increasing the wafer–showerhead distance also reduces the gallium pollution, this time by a factor of around 2. Finally, using AlGaN layers instead of GaN as the channel completely removes gallium pollution, even for composition as low as 5%. It is suggested that in addition to reducing desorption due to alloying, the aluminum precursor may be acting as a getter for this gallium, and so stops it getting to the showerhead. A combination of these different approaches suggests that process conditions can significantly reduce gallium pollution in close‐coupled showerhead reactors.

Obtaining Niobium Nitride on n-GaN by Surface Mediated Nitridation Technique

In this work the n-GaN(1000) surface is used as a source of nitrogen atoms in order to obtain niobium nitride film by a surface-mediated nitridation technique. To this end, the physical vapor deposition of the niobium film on GaN is followed by sample annealing at 1123 K. A thermally induced decomposition of GaN and interfacial mixing phenomena lead to the formation of a niobium nitride compound, which contains Nb from thin film and N atoms from the substrate. The processes allowed the obtaining of ordered NbNx films on GaN. Structural and chemical properties of both the GaN substrate and NbNx films were studied in-situ by surface-sensitive techniques, i.e., X-ray and UV photoelectron spectroscopies (XPS/UPS) and a low-energy electron diffraction (LEED). Then, the NbNx/GaN surface morphology was investigated ex-situ by scanning tunneling microscopy (STM).

Microscopic formation mechanism of Si/Tl5Al1/TiN ohmic contact on non-recessed i-InAlN/GaN heterostructures with ultra-low resistance

In this work, we use Si/Tl5Al1/TiN for a source/drain ohmic contact to demonstrate an ultra-low contact resistance of 0.11 Ω mm (ρc = 2.62 × 10−7 Ω cm2) on non-recessed i-InAlN/GaN heterostructures. The Ti5Al1 alloy was used to suppress the out-diffusion of Al and extract N from the InAlN layer, which aided the formation of ohmic contact by improving the tunneling efficiency of electrons, as we have reported in the past work. A thin Si inter-layer combined with the Ti5Al1 alloy is proposed to further reduce contact resistance. A heavy n-type InAlN layer was obtained through doping with Si atoms to improve the tunneling transport of electrons. Furthermore, the TiN inclusions penetrated into the GaN channel because the in-diffused Si promoted the decomposition of GaN at a high annealing temperature and the in-diffused Ti reacted with GaN. These TiN inclusions provided direct contact with two-dimensional electron gas, offering an additional path for the injection of electrons into the channel. The tunneling and spike mechanism worked alternately to lower the contact resistance at different annealing temperatures (dividing at 900 °C), implying that the joint effect of tunneling and the spike mechanism was initially promoted in the formation of ohmic contact. The mechanism of this Si/Ti5Al1/TiN ohmic contact was fully understood through microscopic and thermodynamic analyses. These results shed light on the mechanism for the formation of ohmic contact in a gold-free metal stack for GaN-based HEMTs.

Impact of Protective Layer Structures on High‐Temperature Annealing of GaN

GaN decomposition at high‐temperature annealing after ion implantation is a huge hurdle to the further maturation of GaN technology. To solve this issue, this work studies the impact of different protective layer structures on high‐temperature annealing of GaN under 1200–1250 °C, including single‐layer structures and double‐layer structures. Single‐layer structures are the SiN layer grown by low‐pressure chemical vapor deposition (LPCVD) and the SiON layer by plasma‐enhanced chemical vapor deposition (PECVD). Double‐layer structures include oxides (Al2O3, SiO2, HfO2) deposited by atomic layer deposition (ALD) on top of low‐stress SiN by LPCVD or SiON grown by PECVD and epitaxial AlN by metal–organic chemical vapor deposition (MOCVD) with sputtered AlN. It is found that either SiO2 with the low‐stress SiN or AlN double capping layers was capable of completely preventing GaN decomposition in the temperature range of 1200–1250 °C. Finally, the protective layer with 20 nm Al2O3 and 300 nm low‐stress LP‐SiN is utilized in the annealing of Si‐implanted GaN high electron mobility transistor (HEMT) structure for activation. The excellent electrical performance are characterized. This work can provide valuable information on the ion implantation and high‐temperature annealing of GaN, which is critical for the further development of gold‐free ohmic contact with GaN devices.

In situ spectral reflectance analysis of the early stages of GaN thermal decomposition

The decomposition of GaN layers have been in situ monitored by a spectral reflectance system. A particular interest is given to the early stages of GaN decomposition. The analysis of the spectral reflectance behavior shows a rapid reflectance increase, which is associated with the formation of gallium metallic layer. After that, reflectance oscillates and damps with the processing time indicating the decomposition of the GaN layer. The transfer matrix formalism and the effective refractive index concept have been combined to understand the experimental recorded reflectance responses during GaN decomposition under H2 flux at high temperature. We have developed an effective refractive index formula taking into account the coexistence of gallium and GaN materials for fitting experimental results. This assumption has been validated as a good tool for the interpretation of the reflectance variations. Our results are confirmed and analyzed based on scanning electron microscopic (SEM) observations and photoluminescence measurements.

Effect of pulsed ion-beam treatment on the electronic and optical properties of GaN epitaxial films on sapphire

GaN films epitaxially grown on sapphire substrates and stored for more than a year under normal conditions were treated using pulsed mixed carbon-hydrogen ion beam. The electronic structure transformations of GaN on sapphire samples were studied using X-ray photoelectron spectroscopy (XPS), optical absorption, photoluminescence and density functional theory (DFT)-based simulations after such treatment. Weak acidification in the form of Ga—N…O was revealed by XPS method for both as-grown and ion-irradiated GaN on sapphire samples. GaN decomposition and Ga-loss effects were not detected after pulsed ion-beam treatment (PIBT), either experimentally or theoretically. It was shown that PIBT promotes a narrowing of the bandgap, changes the vacancy concentration in the surface region, and provides hydrogenation of the surface with the formation of hydroxyl groups in nondefective areas of the surface and near surface layers. PIBT-induced densification of the layers is observed due to annealing of vacancies; no detectable changes in the surface area morphology were found. The weak shift of the valence band edge corresponds to the contribution from areas with more nitrogen defects. The presence of these states in the bandgap could be the possible source of an increase in the electro and photocatalytic activity of GaN epitaxial films.

The effect of annealing on photoluminescence from defects in ammonothermal GaN

Ammonothermal GaN samples with the concentration of free electrons of 1018 and 1019 cm−3 were annealed in a wide range of temperatures (Tann = 300–1400 °C) under atmospheric N2 pressure and under ultra-high N2 pressure conditions to avoid the GaN decomposition. Photoluminescence (PL) studies reveal the YL2 band with a maximum at 2.3 eV before annealing and two new PL bands after annealing at Tann &amp;gt; 600 °C: the OL3 band with a maximum at 2.1 eV and the RL4 band with a maximum at 1.6–1.7 eV. The ammonothermal GaN samples have high concentrations of complexes containing gallium vacancy (VGa), hydrogen, and oxygen. The first-principles calculations suggest that the VGa-3Hi complex is the origin of the YL2 band, while the VGa-3ON complex is responsible for the RL4 band.

Growth of highly relaxed InGaN pseudo-substrates over full 2-in. wafers

A highly relaxed InGaN buffer layer was demonstrated over a full two-inch c-plane sapphire substrate by metalorganic chemical vapor deposition. The InGaN buffer layer was grown on a 100 nm GaN decomposition stop layer with a 3 nm thick high indium composition InGaN underlayer. After thermal decomposition of the underlayer at 1000 °C, a 200 nm thick In0.04Ga0.96N buffer showed 85% biaxial relaxation measured by a high resolution x-ray diffraction reciprocal space map. When used as a pseudo-substrate for the regrowth of InGaN/InGaN multi-quantum wells, the sample showed a 75 nm red-shift in room temperature photoluminescence when compared to a co-loaded GaN template reference. The longer emission wavelength is associated with higher indium incorporation in the InGaN layers from the lessening of the compositional pulling effect caused by compressive strain. Using this technique, a simple red light emitting diode was demonstrated with an active layer growth temperature of 825 °C and a peak wavelength of 622 nm at a current density of 20 A cm−2. This work represents a unique method to relax a III-nitride based layer over a full substrate.

One-Step Fabrication Method of GaN Films for Internal Quantum Efficiency Enhancement and Their Ultrafast Mechanism Investigation.

The third-generation semiconductors are the cornerstone of the power semiconductor leap forward and have attracted much attention because of their excellent properties and wide applications. Meanwhile, femtosecond laser processing as a convenient method further improves the performance of the related devices and expands the application prospect. In this work, an approximate 3 times improvement of the internal quantum efficiency (IQE) and a 5.5 times enhancement of the photoluminescence (PL) intensity were achieved in the GaN film prepared using a one-step femtosecond laser fabrication method. Three types of final micro/nanostructures were found with different femtosecond laser fluences, which could be attributed to the decomposition, melting, bubble nucleation, and phase explosion of GaN. The mechanisms of the microbump structure formation and enhancement of IQE were studied experimentally by the time-resolved reflection pump-probe technique, X-ray photoelectron spectroscopy (XPS), and Raman spectroscopy. Simulations for the laser-GaN interaction have also been performed to ascertain the micro/nanostructure formation principle. These results promote the potential applications of femtosecond lasers on GaN and other wide band gap semiconductors, such as UV-light-emitting diodes (LEDs), photodetectors, and random lasers for use in sensing and full-field imaging.

Study on the Variation of Surface Morphology and Residual Stress Under Various Thermal Annealing Conditions with Bulk GaN Substrates Grown by HVPE

We investigated the effects of different thermal treatment conditions on the surface and internal residual strains of bulk GaN grown by hydride vapor phase epitaxy (HVPE). Thermal annealing was performed at 700–1000 °C for 1–5 h in nitrogen atmosphere. The GaN was characterized by atomic force microscopy, energy dispersive spectrometry, X-ray photoelectron spectroscopy, Raman spectroscopy, and by high-resolution X-ray diffractometer. The experimental results demonstrated that thermal decomposition and oxidation occurred on the surface of GaN when exposed to heat over a long time, or even at a low temperature, as compared to thermal decomposition of GaN in ambient nitrogen. The internal residual stress of GaN was relaxed most effectively when annealing at 900 °C for 3 h, and it was confirmed that the crystal quality is best under this condition. We also confirmed that the effect of annealing was extremely beneficial because native oxide impurities were removed most effectively in this condition. However, Ga metal or oxide could formed due to the occurrence of slight thermal decomposition on the surface.

HAFNIUM AND NITROGEN INTERACTION AT Hf/GaN(0001) INTERFACE

The growth and stability of hafnium films on [Formula: see text]-GaN(0001) surface with native oxide was investigated with X-ray and ultraviolet photoelectron spectroscopy (XPS, UPS). It is shown that hafnium creates a continuous and stable layer on GaN substrate. Thermal treatment at [Formula: see text]C of Hf/GaN system causes decomposition of GaN and reaction of hafnium with atomic nitrogen from the substrate. XPS spectra demonstrate the reaction by a strong shift of the N 1s and Hf 4f lines. An attempt for bringing on the same reaction with molecular nitrogen under pressure of [Formula: see text] mbar was not successful. UPS spectra show a metallic character of the hafnium adlayer in such instances.

Ultraviolet Stimulated Emission in AlGaN Layers Grown on Sapphire Substrates Using Ammonia and Plasma‐Assisted Molecular Beam Epitaxy

Ammonia and plasma‐assisted (PA) molecular beam epitaxy modes are used to grow AlN and AlGaN epitaxial layers on sapphire substrates. It is determined that the increase in the thickness of the AlN buffer layer grown by ammonia molecular beam epitaxy (MBE) from 0.32 to 1.25 μm results in the narrowing of 101 X‐Ray rocking curves (XRCs), whereas no clear effect on the 002 XRC width is observed. It is shown that strong GaN decomposition during growth by ammonia MBE causes AlGaN surface roughening and compositional inhomogeneity, which leads to deterioration of its lasing properties. AlGaN layers grown by ammonia MBE at an optimized temperature demonstrate stimulated emission (SE) which peaks at λ = 330, 323, 303, and 297 nm, with SE threshold values of 0.7, 1.1, 1.4, and 1.4 MW cm−2, respectively. In comparison with these, AlGaN layers grown using PA MBE pulsed modes (migration‐enhanced epitaxy, metal‐modulated epitaxy, and droplet elimination by thermal annealing) show SE with a relatively low threshold (0.8 MW cm−2) at a considerably shorter wavelength of λ = 267 nm.